1. Field of the Invention
The present invention relates to an austenitic Fe—Cr—Ni alloy for use at high temperatures.
2. Description of the Related Art
Austenitic Ni-base alloys containing Cr up to 30 wt %, Si up to 3 wt %, varying amounts of Fe and sometimes additions of R.E.-elements (Rare Earth) have long been used for a variety of high temperature parts up to 1100° C. service temperature. Regarding electric resistance alloys used for heating in industrial furnaces and in appliances, several alloys with varying amount of Ni are standardized in ASTM B 344-83 and in DIN 17470. These standards are not fully compatible as seen from Table 1. There are several commercial resistance alloys using variations on the theme, such as the 37-21 alloy, comprising 37% Ni, 20 to 21% Cr, 2% Si, and the balance Fe, and minor additions of rare earth elements including Yttrium (designated R.E.).
It is an object of the present invention to provide alloy compositions that combine the lower cost of a Ni content in the range, if possible, close to NiCr 30/20, i.e., 30 wt % Ni and 20 wt % Cr, with
i) a good hot form stability; and
ii) an oxidation resistance; and
iii) a relatively high electrical resistance and low change of resistance (Ct); of a higher Ni content alloy such as NiCr 60/15.
TABLE 1Summary of ASTM and DIN Standards for resistance eCr(Fe) alloysDIN *) 1774- W. .Cr Ni+C Fe Al Si Mn C Oth Note p CtNr. o er (pQm) 900° C . NiCr 2.4 19- >75 <1,0 <0, 0,5- <1, <0, R.E 1,12( 1, 14 80 869 21 3 2,0 0 15 .1,08) 20NiCr 2.4 ...30 >60 <5,0 <0, 0,5- <1, <0, R.E 1,19( 1,27 ) - 70 658. .3 2,0 0 10 .1,16) 30 .~NiCr 2.4 14- >59 19,0 <0, 0,5- >2, <0, R.E 1,13( 1,23 -) 60 867 19 -3 2,0 0 15 .1,11) 15 ...25,0 .NiCr 1.4 20,028.,0 ba1 2,00 <1, <0, Only 1,04 1,28 30 860..- --5 20 17470 20 22,031,0 3,00NiCr 1.422,0 19,0 bal 1,5- <2, <0, Only 0,95 1,24 25 843 --2,5 00 20 17470 20 25,022,0ASTM B 344-8380Ni 19- bal. <1,0 0,75 <2, <0, S<O,O 1,081 ″ , 21 -5 15 1 :) 20Cr 1,7560Ni 14- >57 0,75 <1, <0, S<O,O 1,122 , 18 -0 15 1 16Cr 1,75 .. 35Ni 18- 34- bal 1,0- <1, <0, R.E S<O,O 1,014. , 21 37 3,0 0 15 .1 20Cr* Maximum 1% Co
In general, the maximum operating temperature and lifetime of an alloy increases with increased Ni-content, but several other elements have great impact on these properties as well. All of these alloys form a protective oxide layer composed of mainly Cr2O3, and in case of Si additions also SiO2 to some extent. Smaller additions like rare earth elements have been used to further enhance the properties of the oxide layer, and several patents advise additions to provide a material with good oxidation life, see, e.g., EP 0 531 775 and EP 0 386 730.
In addition to good oxidation there is also a demand for good hot strength. In case of electric elements, the cost for hangers and support systems can be reduced if the material is strong enough to support its own weight, and therefore to maintain its shape at operating temperature.
For use as electric elements, the relatively high resistivity and low Ct=Rhot/Rcold ratio of resistance change from room temperature to working temperature is an important parameter. In general, the higher the Ni, the higher the resistivity and the lower the Ct factor.
Addition of elements such as Mo and W up to levels of several wt % are known to enhance the mechanical properties at high temperatures, but they are expensive and are therefore not desirable additions in applications where cost is important.
In a wide range of open coil electric resistance heating elements, NiCr 60/15 and NiCr 30/20 type (DIN) or 60 Ni, 16 Cr and 35 Ni, 20 Cr (ASTM) alloys are used. From a cost point of view, the NiCr 30/20 or 35Ni, 20Cr type is preferred due to their lower content of expensive Ni. In applications where the watt density and therefore the element temperature are high, the oxidation life of alloys with this level of Ni has up to now been insufficient. At the same time, the mechanical properties at working temperatures have to be within acceptable limits.